1. Field of the Invention
The present invention relates to a silicon nitride sintered body having high thermal conductivity as well as excellent electric insulation properties under high temperature, and the method of producing the same.
2. Description of Related Art
Silicon nitride sintered bodies possess excellent mechanical characteristics such as strength and toughness, and other various excellent characteristics including abrasion resistance, oxidation resistance, electric insulation properties and so forth, so that they have been generally used as a structural material for heat engines such as diesels and gas turbines. However, they have not been used for members having radiation properties (for instance, circuit substrates) since their thermal conductivity characteristics are inferior to those of sintered bodies of BeO, AlN, SiC and so forth.
However, the present applicant has tried various modifications under the consideration that silicon nitride sintered bodies may be preferably used for circuit substrates and so forth if their thermal conductivity characteristics can improve, while making use of their excellent mechanical characteristics and electric insulation properties.
As in, for example, Japanese Patent Application Publication No. 3-218975, it has already been disclosed that silicon nitride sintered bodies having both excellent mechanical characteristics and preferable thermal conductivity characteristics may be obtained by controlling the size of crystal grains of a polycrystalline substance of silicon nitride as well as the content of Al as an impurity.
Additionally, as in Japanese Patent Application Publication No. 9-30866, it has recently been disclosed that silicon nitride sintered bodies having preferable mechanical characteristics and high thermal conductivity may be obtained by forming the crystal grains of a polycrystalline substance of silicon nitride as a composite of rough grains and fine grains and by regulating the ratio of rough grains. The above-mentioned silicon nitride sintered bodies having excellent thermal conductivity may be preferably used as the circuit substrates that are required to radiate heat smoothly from an element and to completely insulate both sides of a substrate and inter-wires on and in the substrate. Examinations have been carried out on, e.g., semiconductor modules and the like wherein a metal circuit is provided with the silicon nitride sintered body as a substrate and a semiconductor element is further placed thereon.
However, although the above-mentioned silicon nitride sintered bodies show sufficient electrical insulation properties under room temperature, their electrical insulation properties often decline sharply under high temperature, so that the bodies have been insufficient as, e.g., on-vehicle circuit substrates that need electric insulation properties under high temperature of 100xc2x0 C. or higher. Particularly, when they are used as a circuit substrate for a semiconductor device, such as a thyristor, which controls a large current, they must have higher electrical insulation properties than average circuit substrates, so that the above-described problem becomes very apparent.
Specifically, a silicon nitride sintered body has not yet been found that has preferable characteristics as a circuit substrate used under high temperature. Thus, a silicon nitride sintered body has been in demand that satisfies both thermal conductivity characteristics and electrical insulation properties under high temperature.
It is an object of the present invention to provide a silicon nitride sintered body having high thermal conductivity and excellent electrical insulation properties under high temperature, and a method of producing the same.
Specifically, according to the present invention, a silicon nitride sintered body is provided which includes silicon nitride crystalline grains and a grain boundary phase and contains Mg at 0.3 to 10 wt. % in terms of oxide. The sintered body has a thermal conductivity of 60 W/mK or higher and has an electric resistivity of 1E+13 xcexa9cm or above at 125xc2x0 C.
The silicon nitride sintered body of the present invention preferably has an electrical resistivity of 1E+11 xcexa9cm or above at 400xc2x0 C., and the percentage content of a simple substance of Al is preferably 0.25 wt . % or less. Moreover, according to the present invention, a circuit substrate is provided that includes the silicon nitride sintered body mentioned above.
Furthermore, according to the present invention, a method of producing a silicon nitride sintered body is provided that includes the steps of compacting molding materials including silicon nitride powder, a Mg compound and a sintering aid, and sintering the compact under a nitrogen atmosphere at 1,800 to 2,000xc2x0 C. After preparing the molding materials to contain Mg at 0.3 to 10 wt. % in terms of oxide and keeping a constant temperature for 0.5 hours or longer in the temperature range of 1,400 to 1,700xc2x0 C., the temperature is raised to the sintering temperature.